1. Field of the Invention
The present invention relates in general to rare gas filled lamp constructions, and in particular arc discharge lamp constructions employing a mercury capsule for dispensing mercury into the lamp envelope. More particularly, the present invention relates to an improved means for the support and positioning of the mercury capsule in the lamp envelope.
2. Background Discussion
In the early construction of mercury filled lamps, mercury was introduced into a lamp as a liquid. Mercury doping is still commonly carried out by dispensing machines. During the exhaust cycle prior to lamp sealing, the machines dispense beads of mercury, from a reservoir, through a needle directly into the lamp. Dispensing mercury even by machine can be difficult. Mercury may be spilled during normal handling, because of machine error, or during machine repair. Spilled mercury shatters and rolls, but is not absorbed, making cleanup difficult.
With a greater stress on environmental protection, mercury is more frequently being enclosed in a capsule. The capsule is inserted in the lamp, and forced to open once the lamp is closed. The mercury capsule then holds and controls the mercury during assembly and thereby helps eliminate the handling and machine related problems.
The mercury capsule is typically inserted into the lamp during manufacture as an appendage of the lamp filament or lamp seal structure. In this regard, by way of example, refer to U.S. Pat. Nos. 3,913,999 to Clarke; 4,534,742 to Grossman, et al; and 4,539,508 to Mulder, et al. The above-referenced patents show different arrangements, all of which suspend the mercury capsule in one manner or another from the electrode structure, basically at or close to the lamp seal area. Once the lamp is closed, the mercury is released by heating the capsule during light up. The heating may come from the nearby filament or from a separate external heating source, such as an RF heat. Heat causes the mercury pressure to increase in the metal capsule. With sufficient pressure the capsule opens, expelling the mercury safely into the sealed lamp.
A further illustration of a standard mercury capsule support is shown in the prior art drawing of FIG. 1. FIG. 1 illustrates a standard fluorescent lamp with a mercury capsule. The view of FIG. 1 is a fragmentary view of one end of a fluorescent tube illustrating the tubular envelope 10 having a flame seal area at 12 and further illustrating an electrode at 14. FIG. 1 shows the mercury capsule 16 being supported as an extension of the structure of coil 14 by means of a support wire 18.
In connection with the lamp construction of FIG. 1, the fluorescent lamp typically uses a relatively soft glass such as type 080 soft glass, requiring approximately 800.degree. C. to seal a 3 to 4 millimeter wide area such as shown at the press-seal area 12 at FIG. 1. Thus, a relatively low heat level is required to seal the envelope, and the low heat does not reach the mercury capsule disposed on the support wire 18, on the opposite side of coil 14.
Lamps used in high temperature applications; however, use high temperature glasses, and sealing areas can be as long as 23 millimeters. Sealing may also involve temperatures reaching as high as 2000.degree. C. The extreme heat travels up the mount and glass and can cause the mercury capsule to prematurely open during the press-seal operation when a mounting arrangement typical of the prior art shown in FIG. 1 is used. Premature rupture of the mercury capsule may result in improperly dosed lamps, but more importantly, may be hazardous to plant and personnel. There is then a need to safely provide mercury doses in lamps, and in particular, in lamps made with high temperature seals.